That's great Matt.
Historically, limiting stereo width in the low bass region was an important step in mastering for technical reasons when everything was released on vinyl, and still is if vinyl is the target medium. High levels of side component (width) in the bass region produce vertical cutting head undulations which can cause the playback stylus to jump out of the groove. That limitation goes away completely with digital media, but there can still be sonic reasons for wanting to modify width of the low bass besides trying to make things sound like LPs. It's a technique often used in bass heavy music styles to maximize bass loudness and impact. To a lesser degree, managing low frequency width may also be important for optimizing playback on satellite systems which share a single subwoofer as opposed to systems with stereo woofers. In that case the low frequency rage is summed to a single woofer and the difference components in that range tend to cancel. In our case, I can see how it could help manage the often problematic bass region with some AUD tapes.
Like anything else, be judicial, use your ears and don’t go overboard in mono-ing the bass. Low frequency phase differences contribute strongly to spatial envelopment (the sensation of being in the environment in which the recording was made) and externalization (sound seeming to emanate from the world around you, rather than from ‘inside your head’). When it behaves, I prefer slightly wider that just ‘wide enough’ bass because I'm somewhat of an envelopment junkie, but if narrowing it helps improve things in the bass or in the overall, I’m all for making adjustments that improve things.
There are some plugins, including free ones I think, which do similar multiband stereo width processing simply, in one process. If you do this regularly it may be worth your time checking them out since they eliminate the need for you to do the track duplication, crossover filtering, and summing after processing. The terminology used varies- some call it mono-izing the bass region, some may refer to it as reducing the Side component and increasing the Mid component of the bass region.. it's the same thing. You can also do this manually another way by converting the stereo signal to Mid/Side, equalizing the Mid and Side components differently, and then recombining them. Reducing the level of the Side-signal while introducing a complementary, offsetting boost in the Mid-signal has the same effect of narrowing the stereo width. Where in the frequency range you make the boost/cut determines at which frequency range that width adjustment takes place- it basically gives you as many stereo width adjustment bands as you have in the equalizer. I see more and more stereo plugins (EQ, compression, etc) offering a way to switch from Left/Right mode to Mid/Side mode which takes care of the stereo>M/S>stereo conversion all within the plugin. I use Samplitude which has stereo width control functions built in that feature mulitband options allowing adjustment via one, two or three separate bands.
One clarification- I don't mean to be pendantic, and only mean to help clarify the terminology here- but the correct term for splitting the audio regions up and processing them this way is mulitband processing as opposed to parallel processing. The difference is that multiband splits the audio into two or more frequency ranges, does separate processing on each rage, then combines them again. Parallel processing does it’s thing on a full frequency copy (mult) of the audio and mixes that with the original. Mulitband processing it's most often used for compression, or things like stereo width adjustment you are doing. EQ is already multiband by nature.
